1. Field of the Invention
The present invention relates to a device and a method for controlling an idle speed by opening and closing a bypass passage connecting portions of an intake passage upstream and downstream of a throttle valve.
2. Description of the Related Art
An idle speed control device is used to adjust a degree of opening of an idle speed control valve provided in the bypass passage to control the idle speed to a set value. In a known idle speed control device, a feedback control and a learning control is carried out to control the idle speed to a target value. The feedback control is carried out when a feedback condition occurs in which, for example, an idle switch is turned ON and the vehicle speed is 0 km/h, so that the degree of opening of the idle speed control valve is changed to bring the idle speed to the target value. The learning control is carried out when a learning condition occurs in which, for example, the feedback control is carried out and the temperature of the cooling water is higher than 80.degree. C., so that a learning value of the degree of opening of the idle speed valve, by which the idle speed is brought to the target value, is determined. The degree of opening of the idle speed control valve is set to the learning value when the feedback condition is not satisfied and an open-loop control is carried out.
In another known device, the degree of opening of the idle speed control valve is gradually brought to the learning value when the feedback condition is not satisfied. However, if the learning value is abnormally small, the degree of opening of the idle speed control valve also becomes too small, so that the number of engine revolutions is reduced, and thus the engine tends to stop when the accelerator is released during running of the vehicle to close the throttle valve and decelerate the vehicle.
In a further known device, when the feedback condition is not satisfied, the degree of opening of the idle speed control valve is maintained at the value at which the degree of opening for the feedback control was carried out until that point. However, if the degree of opening of the idle speed control valve is too small when the feedback control is stopped, the engine will easily stall, as in the above-mentioned device.
Japanese Unexamined Utility Model Publication No. 60-188840 discloses a construction in which, when a correction coefficient of the feedback control is a negative value, the correction coefficient is fixed to zero, so that engine stalling is prevented when the open-loop control is stopped and feedback control started.